This invention relates in general to inkjet printheads and, more specifically, to control in the directionality of ink drops ejected from a printhead in order to improve image quality. More particularly, the invention relates to a method of compensating for defects in an inkjet printhead having at least one nozzle to correct misdirection of ink drops ejected from the nozzle.
Without limiting the scope of the invention, its background is described in connection with inkjet printers, as an example.
Modern color printing relies heavily on inkjet printing techniques. The term xe2x80x9cinkjetxe2x80x9d as utilized herein is intended to include all drop-on-demand or continuous inkjet printer systems including, but not limited to, thermal inkjet, piezoelectric, and continuous, which are well known in the printing industry. Essentially, an inkjet printer produces images on a receiver medium, such as paper, by ejecting ink droplets onto the receiver medium in an image-wise fashion. The advantages of non-impact, low-noise, low-energy use, and low cost operations, in addition to the capability of the printer to print on plain paper, are largely responsible for the wide acceptance of inkjet printers in the marketplace.
The printhead is the device that is most commonly used to direct the ink droplets onto the receiver medium. A printhead typically includes an ink reservoir and channels, which carry the ink from the reservoir to one or more nozzles. Typically, sophisticated printhead systems utilize multiple nozzles for applications such as high-speed continuous inkjet printer systems, as an example. Continuous inkjet printhead device types include electrostatically controlled printheads and thermally steered printheads. Both printhead types are named according to the means used to steer ink droplets ejected from nozzle openings.
It is well known in the art of inkjet printing that image quality suffers from a failure to accurately control the direction from which ink drops exit the printhead. Variations in the direction of ink drops ejected from a given nozzle from a desired direction of ejection (usually perpendicular to the printhead surface) can occur due to changes in the nozzle during operation, as a result of manufacturing defects present before operation, or both. In most instances, repairs are too difficult and costly, resulting in scrapped parts and decreased manufacturing yields. Accordingly, a cost effective way of increasing printhead lifetimes and printhead production yields would be advantageous.
For any given nozzle, the direction of the exiting ink drop stream is controlled by the physical characteristics of the nozzle. Where misdirection occurs, the ink drops can produce printing artifacts such as random placement errors between subsequent drops from a single nozzle or placement errors of drops from one nozzle with respect to those from another nozzle. Variations in the direction of ink drops ejected from a given nozzle may occur over a variety of time scales. For example, in Bubble Jet printheads, made by Canon Company, rapid variations may occur when bubbles nucleate randomly on the surfaces of heaters, causing random variations in the velocity and direction of ejected ink drops from each nozzle. Variations in the direction of ejected ink drops may also be caused by sources external to the inkjet printhead such as, for example, vibrations of the inkjet printer. It is difficult or impossible to correct such random variations in the direction of ejected ink drops, which typically change rapidly with time.
In other cases, factors causing deviation of the direction of ejected ink drops from a desired direction can occur slowly over a long period of time. Such slowly changing variations may arise, for example, from gradual changes in the material properties of the nozzle, such as changes in the stress of the materials comprising the nozzle or surrounding the nozzle openings, from changes in the resistance of heater materials during operation, or from wear of nozzle materials during operation.
In still other cases, factors causing deviation of the direction of ejected ink drops from a desired direction can be essentially permanent. Deviations caused by manufacturing defects in nozzles, for example defects which alter or vary the shape of the nozzle openings, are essentially permanent. Permanent deviations may also arise after a period of time of operation of a nozzle. For example, a piece of material may become permanently chipped away from a portion of a nozzle after a period of time of operation, or a piece of material may lodge permanently within a nozzle during operation.
Thus, it is desirable to compensate for slowly changing variations in the directionality of ejected ink drops. For slowly changing variations, compensation may be needed from time to time during operation. It is also desirable to compensate for permanent changes in the directionality of ejected ink drops in order to improve image quality and increase manufacturing yield. Compensation cannot be applied before operation of the nozzles, since it is generally not possible to predict the direction and magnitude of deviations in the direction of ejected drops for a particular nozzle, which occur after operation. Compensation applied after or during operation of nozzles is herein referred to as active compensation.
Substantial effort has been directed toward active compensation for slowly changing variations in the direction of drop ejection for drop on demand printers, as discussed and illustrated, for example, in U.S. Pat. No. 4,238,804, assigned to Xerox Corporation, and U.S. Pat. No. 3,877,036, assigned to IBM, which teach measuring the position of ejected ink drops and compensating for variations from the ideal direction by electrostatic means. While such electrostatic deflection can be used to direct ink in a desired direction, as is well known in the art, electrostatic deflection in these cases adds mechanical complexity. Also, correction techniques of this type are largely ineffective in cases where large variations in the direction of ejected ink drops occur.
U.S. Pat. No. 5,592,202, assigned to Laser Master Corporation, teaches an electronic means to correct inaccuracies in ink drop placement by advancing or retarding the time of a drop-on-demand actuation pulse. However, this method does not correct variations in both of the directions of ink drop ejection in a plane perpendicular to the direction of drop ejection, as it is more suited to adjusting ink drop placement only in the scan direction of the printhead. Moreover, not all printhead circuits can be easily adapted to control the firing times of individual ink drops, since the firing pulses may be derived from a common clock.
U.S. Pat. No. 5,250,962, assigned to Xerox Corporation, teaches the application of a moveable vacuum priming station that can access groups of nozzles to remove entrained air in one or more nozzles. Although entrained air is known in the art to cause variations in the direction of ink drop ejection, it is only one of many mechanisms causing variations. Also, entrained air principally refers to failure of the ink to fill the printhead, not to a change in the head itself. Removal of trapped air serves to restore the nozzle to its original condition, but does not alter the physical characteristics of the nozzle.
Other prior art techniques for achieving compensation include the selection of one nozzle among a plurality of redundant nozzles for printing a particular imaging pixel, the preferred nozzle having favorable ink drop ejection characteristics. However, redundancy selection techniques of this type are complex in nature and require substantial real estate space on the printhead form factor to implement. Such methods also increase cost and/or reduce productivity.
In the case of continuous inkjet printheads using electrostatic steering of drops, as in the current generation of commercialized continuous inkjet printheads, for example those manufactured by Scitex Corp., compensation for variations in the direction of ejected ink drops from an ideal direction can be accomplished by electrostatic means; and in this case, additional mechanical complexity is not required, since the means of printing itself is based on electrostatic deflection and the required hardware is already in place. Printheads of this type produce electrically charged ink drops, which are deflected using a charged electrode at each nozzle. The electrode voltage is set to one of two discreet values (for example, either 100 volts or 0 volts) each time a drop is ejected, causing drops to be deflected either in a printing direction (for example, in the case the voltage is 100 volts), or into a gutter (for example, in the case the voltage is 0). To correct for slow or permanent deviations of the direction of ejected drops from a particular nozzle, the voltage corresponding to printing at that nozzle might be set, for example, to 110 volts. The use of electro-static techniques such as these, however, requires additional voltage control hardware.
In the case of continuous inkjet printheads using thermal steering of drops, an electrode apparatus is not already in place, and other means of correction are desired to correct for the effects of slow variations in direction of ink drop ejection, as well as for permanent manufacturing defects.
Accordingly, a need exists for a cost effective method of correcting defects in inkjet printheads to permit compensation in the direction of ink drops ejected from the nozzles. A means of increasing manufacturing yields by permitting active compensation for ink drop ejection misdirection from a nozzle would provide numerous advantages.
The present invention provides a method of correcting misdirection of ink drops ejected from the nozzles of an inkjet printhead which occur from time to time after the manufacture of the printhead and/or during operation of a printhead having at least one nozzle with heater elements to direct ink drops ejected from the nozzle. With the present invention, thermally steered printheads that would normally be discarded due to defects that cause ink drop misdirection can be repaired rather than discarded, and thermally steered printheads that fail due to the behavior of one or more nozzles which, after operation, eject ink drops in a direction which is not the desired ink drop ejection direction can be repaired without removal from the printer.
Accordingly, disclosed in one embodiment is a method of compensating for the effects of defects in an inkjet printhead to permit control in the direction of ink drops ejected from a nozzle of the printhead. Initially, the printhead is tested to determine its ink stream directionality onto a receiver medium, such as paper. Thereby, the amount of misdirection from a nozzle of an inkjet printhead is thus quantified, as is well known in the art.
The method comprises the steps of immersing the heater elements surrounding the nozzle in an electroplating solution and applying a voltage differential measured with respect to the electroplating solution to at least one of the heater elements in order to add electroplated material to that heater element, or to remove electroplated material from a heater element to which electroplated material had been previously added. In one embodiment, an ink drop deviation angle from the desired vertical direction for ink drops exiting one of the nozzles is calculated and a voltage differential is applied to one of the heater elements in order to cause a deflection of the ink drop stream in a desired direction. The electroplated material acts to compensate for any misdirection of ink drops out of the printhead nozzles. Since the heater elements may include both heaters and heater electrodes located at numerous locations around the nozzle, electroplated material can be applied at numerous locations around the nozzle.
For example, if ink drop deviation to the left of the desired vertical direction is desired, a voltage differential can be applied to a right heater electrode of the nozzle in order to deflect the ink drop stream to the left; whereas, if ink drop deviation to the right of the desired vertical direction is desired, a voltage differential can be applied to the left heater of the nozzle.
The step of applying the voltage differential can be performed by applying a voltage differential to a heater element having a value for which electroplating occurs in order to establish an increased thickness of electroplated coating across the area spanned by the heater element. Alternatively, the step of applying the voltage differential can be performed by applying a voltage having a value for which electroetching occurs in order to establish a reduced thickness of electroplated coating across the area spanned by a heater element having been previously subjected to electroplating. Electroetching of material deposited by means other than electrodeposition is also possible. Additionally, the time of exposure to the voltage differential can be varied in order to vary the final characteristics of the electroplated coating. The electroplated coating acts to compensate for any misdirection of ink drops out of the printhead nozzles.
According to another embodiment, disclosed is an inkjet printhead with integral compensation for misdirection of ink drops ejected through at least one nozzle of the printhead. The inkjet printhead comprises a nozzle cavity adapted for facilitating the flow of ink from an ink reservoir. The inkjet printhead also comprises a membrane predisposed about the nozzle cavity to create a resistive barrier against ink flow. The membrane includes a nozzle opening through which ink drops are ejected.
In accordance with yet another embodiment, the inkjet printhead further comprises heater elements predisposed to direct the flow of ink drops through the nozzle opening. The heater elements comprise heater electrodes and heaters. The heaters include a right heater and a left heater, which are predisposed about the nozzle opening. In addition, the heater electrodes further include one or more lower left electrodes and one or more lower right electrodes. The heater electrodes are electrically coupled to the heaters so as to have the same electrical voltage as the heater and predisposed about the nozzle opening. As such, the heater electrodes and heaters are separated by the membrane.
Technical advantages of the present invention include a cost effective method of compensating for the effects of defects in inkjet printheads that would otherwise result in misdirection of ink drops ejected from the nozzles. As such, printing artifacts caused by irregularities in the ink drops landing onto a receiver medium are eliminated.
Other technical advantages include increases in the useable life of printheads during operation which otherwise would fail due to changes in the direction of ink drop ejection from one or more nozzles from time to time during operation of the printheads and increases in manufacturing yields as printheads that would be typically discarded can be repaired and used.